1. Field of the Invention
The invention proposes a circuit to offer a simple, lumped-element circuit, for use in a monolithic microwave integrated circuit.
2. Description of the Prior Art
N. Marchand published in 1944, "Electronics, " Volume 17, p.12, that a device similar to a transformer that can supply phase-differential but with equal amplitude signal, and simultaneously convert impedance. This is the so-called balanced line-to-unbalanced line transition, shortened to Balun. Later on, S. A. Mass reported in 1992, "Microwave Mixers" second edition, that balun can be widely applied to microwave frequency mixers, phase-differential detector, frequency transformer, and compensated frequency transformer. In addition, K. Tilley et al. reported in 1994 "Electronics Lett." Volume 30, No.3, pp. 176-177, that balun can also be used in the design of progressive amplitude and antenna transmission circuits.
Earlier balance-to-unbalance line transitions use coaxial line structure, or use one or multiple quarter bandwidth combination transmission lines. After several decades' development, balanced line-to-unbalanced line transition balun) has evolved many varieties, which can be classified as the following five types:
1. Broadside coupled-lines type, published by C. Y. Ho in 1985, "Microwave & RF " pp.99-102 and repeatedly covered by other researchers. PA0 2. Marchand type, covered by J. H. Cloete in 1979 at European Microwave Conf. Proc., p. 480 and also covered by other researchers extensively. PA0 3. Double-Y junction type, covered by V. Trifunovic et al. in 1991, "Electronics Lett. " Volume 27, No. 10, pp. 813-815. PA0 4. Active type, as covered by J. Staudinger et al. in September of 1993's, "Microwave Journal" pp. 119-126. PA0 5. Lumped element 180.degree. hybrid, or lumped type, high/low-pass balun, covered by J. A. Eisenber et al. in September of 1992's, "Microwave Journal, pp. 123-131.
The former three balanced line-to-unbalanced line transitions(Balun) are still designed with distributed transmission lines, which is a non-planar circuit design. At lower microwave frequencies (for example 2.0 GHz or below), this type of balanced line-to-unbalanced line transition becomes bulky in size, and unsuited for designing the monolithic microwave integrated circuit. The active type (fourth type) has many problems in the areas of stability, intermodulation, and impedance match. The fifth type of balanced line-to-unbalanced line transition takes 180.degree. hybrid circuit and uses lumped elements to replace a quarter wavelength transmission line. This type of balanced line-to-unbalanced line transition tends to produce larger phase error when it gets farther away from the center frequency while lumped type high/low-pass balun uses high-pass/low-pass filter and diplexer theories in design, but it does not satisfy the phase and frequency match simultaneously.
Balanced-line to unbalanced-line transition (balun) circuits can be widely applied to microwave circuit and antenna applications, microwave frequencies have a similar function to the low frequencies in a transformer except that their structures are difficult to be made into a two-dimensional form. It is usually transmitted by two-types of transmission lines, Hwann-Kaeo Chiou et al. in November of 1995's, "Electronics Lett." Volume 31, No. 24, pp. 2113-2114 and other researchers have covered in regard to the transition between coax/cpw/slot/cps. The transition of these lines invariably required a quarter wavelength line, which causes the balanced-line to unbalanced-line transition(balun) to be a bulky two-dimensional or even three-dimensional structure. Their model characteristics cannot be easily simulated; therefore, in design, it usually requires experience to fine-tune it. The aforementioned difficulties make applying the balanced-line to unbalanced-line transition to the design of monolithic microwave integrated circuit difficult. Microwave balun design applied in monolithic microwave integrated design often uses a 180.degree. magic T-circuit, or an active balanced-line to unbalanced-line transition. Although the latter has a compact advantage there are many shortcomings, such as narrow frequency bandwidth, requiring D.C. bias, different impedance level, and full control of noise figure effect.